How Do Fossils Form?

Video Transcript

"Hi I'm Steve Jones and I'm going to explain how fossils form. First of all what are fossils? Fossils are either the remains of animals or they're signs left by animals. Now signs being it could be imprints of a foot say of an animal's footprint in mud, something of that kind but generally most of the fossils we find we find in rock layers which have been formed at the bed of a lake or a sea so if you imagine the rock layers forming upwards then the fossils are found within that and depending on where they are within the rock sequence you can tell how old the fossil is. Now obviously once the sea disappears and this turns into rock it won't be under the sea any more but it will tell us exactly which sequence these objects came in. Now the question is how do they form and what are they? Well for example shellfish very often fossils would be shells, the shells of snails, the shells of small sea creatures like mussels and so on. That's very often what they are. The other thing they could be are worm casts. So for example a worm will burrow down and then burrow up and very often you'll find these U shaped worm casts within the mud and when it forms rock you can actually see them. This helps you to tell which way the rock has been bent and so forth so that is typically the way you would have a worm cast as evidence of the existence of a marine animal. This would be a kind of filtering worm. Now that is just a small hole in the ground but that hole will fill in with other material and therefore, the two materials will look different and that is why this is only evidence because there is no animal left, the animal died and disappeared but the hole is still there and it was filled in by lighter colored material and you can see that material. So this is the basic way that fossils form."